Where HSR excels is in being able to service several city pairs on a single line. The Northeast Corridor is a great example: you can have a single train service all pairs of {Washington, Baltimore, Philadelphia, NYC, Boston} at negligible extra expense in time and money. Thus you can add in stops like Providence, which don't generate all that many trips on their own, simply because it is literally on the way to where you're going.
The problem with the US Midwest is that there's a lot of these third-tier cities, but you have to go out of your way to connect them. Who wants to go between Columbus and Indianapolis--the highway between the two cities is pretty empty. Even worse, the few collinear city triples you can come up with all completely miss the primary destination of the entire region, Chicago. If you want to go from Chicago to Columbus, Indianapolis, and especially Dayton, are not on your way.
> Who wants to go between Columbus and Indianapolis--the highway between the two cities is pretty empty.
This strikes me as the main problem with the mindset around HSR. Consider e.g., Madison, Wisconsin.
Milwaukee would have a much more vibrant and dynamic ecosystem if undergrads/PhD students/professors could get to/from meetings with investors in MLK/CHI and be back for afternoon classes. Or continue internships/meetings during the school year.
There are enormous inefficiencies (and corresponding brain drain) caused by the fact that the region's center of innovation is 1+ hours by car away from the region's center of finance.
An MLK <-> CHI <-> Madison rail line would change the dynamics of the entire state's economy. Simply counting cars on the roads between those cities in the present ignores how HSR changes the entire dynamic in a very qualitative way.
The requirement for colinearity seems like something you made up. And, you don’t gauge travel demand for one mode based on another. They didn’t build the Golden Gate Bridge because there were many people swimming to Marin. Two well-connected cities with robust economies will enjoy strong travel demand.
There are many things that factor into how likely HSR is to be used. Intercity travel demand is one of the chief metrics, but you also have to consider the quality of intracity public transport. Indianapolis and Columbus as a city pair simply does not score very well on this metric; you'd put the billions of dollars investment to much better use elsewhere.
The point about collinearity is that you can make a strong demand by combining many individually weak demands. And this is the problem in the Midwest: the strong demand is Chicago, and it has a lot of moderate demand endpoints, such as St. Louis, Minneapolis/St. Paul, Indianapolis, Cleveland, Detroit. But the real strong city pair is Chicago/NYC, which is already at the extreme margins of acceptable time, which means substantial detours to hook up the lesser demand centers is going to kill ridership.
The problem with the US Midwest is that there's a lot of these third-tier cities, but you have to go out of your way to connect them. Who wants to go between Columbus and Indianapolis--the highway between the two cities is pretty empty. Even worse, the few collinear city triples you can come up with all completely miss the primary destination of the entire region, Chicago. If you want to go from Chicago to Columbus, Indianapolis, and especially Dayton, are not on your way.